Heat and mass transfer with chemical reaction

3 Heat and mass transfer with chemical reaction 

The general approach in the evaluation of nonisothermal internal effectiveness factors is analogous to that outlined for determining isothermal rj. However, since substantial temperature 

The analytical integration of the differential equation for diffusion with chemical reaction in a catalyst particle is achievable just for first-order reactions. A generalized modulus has been proposed for extending the use of the // expression in Equations 2.61 and 2.64a to any type of rate expression [17], at least approximately. For irreversible nth order reactions, the generalized modulus for a sphere becomes dependent on the exterior surface concentration  

For this reason, both a concentration and a temperature profile are required to describe the conditions at each point within the porous particle, before (-R )pcan be determined. In the case of exothermic reactions, T at any point within the particle may be higher than Ts hence, depending on the relative significance of ACa and AT, the global rate -Ra)p may be higher than the rate at surface conditions ( Ra)s and situations where rj I may be encountered. 

A relationship may easily be derived between intraparticle temperature and concentration differences (AT and AQ) just by considering a boundary surface enclosing some section or all of an arbitrary porous structure as the starting point. Under steady-state conditions, the diffusion of reactants across this boundary surface is equal to the global rate of reaction within the boundary surface. Then, the heat released by reaction within the surface can be expressed in terms of the diffusion rate across the boundary  

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In the last decade we have performed some thousands of experiments in packed adiabatic tubular reactors (CO oxidation), however, we have never observed oscillations. If on certain catalysts (e.g., Pt/Al203) the oscillation are caused by the kinetic mechanism then, apparently, the interactions of heat and mass transfer with chemical reaction suppress the occurence of periodic activity in tubular reactors. 

COUPLED HEAT AND MASS TRANSFER WITH CHEMICAL REACTION IN BATCH REACTORS... 

Stangle, G.C. and Aksay, LA. (1990) Simultaneous momentum, heat and mass transfer with chemical reaction in a disordered porous medium Application to binder removal from a ceramic green body, Chem. Eng. Sci. 45, 1719. 

In this paper only isothermal simulations have been conducted to show the important features of the model to describe mass transfer with chemical reaction. In many industrial processes, distillation, reactive distillation and some absorption processes, heat effects play an important role and therefore cannot be neglected. These effects will be discussed in Part II. 

Astarita, G. Mass Transfer with Chemical Reaction, Elsevier, Amsterdam, 1967. Erank-Kamenetskii, D. A. Diffusion and Heat Transfer in Chemical Kinetics, Plenum Press, New York, 1969. 

In what follows, the preceding evaluation procedure is employed in a somewhat different mode, the main objective now being to obtain expressions for the heat or mass transfer coefficient in complex situations on the basis of information available for some simpler asymptotic cases. The order-of-magnitude procedure replaces the convective diffusion equation by an algebraic equation whose coefficients are determined from exact solutions available in simpler limiting cases [13,14]. Various cases involving free convection, forced convection, mixed convection, diffusion with reaction, convective diffusion with reaction, turbulent mass transfer with chemical reaction, and unsteady heat transfer are examined to demonstrate the usefulness of this simple approach. There are, of course, cases, such as the one treated earlier, in which the constants cannot be obtained because exact solutions are not available even for simpler limiting cases. In such cases, the procedure is still useful to correlate experimental data if the constants are determined on the basis of those data. 

TABLE 27-3 Numerical Results for Coupled Heat and Mass Transfer with First-Order Irreversible Exothermic Chemical Reaction in Porous Catalysts with Rectangular Symmetry"... 

Steady-state analysis of coupled heat and mass transfer with multiple chemical reactions in the pores of a catalytic pellet is based on simultaneous solution of the mass transfer equation ... 

Mann.R. "Heat and mass transfer in exothermic gas absorption". (Proceedings of NATO AS on "Mass transfer with chemical reaction in multiphase systems", Turkey, 1981). 

Correlations of heat and mass-transfer rates are fairly well developed and can be incorporated in models of a reaction process, but the chemical rate data must be determined individually. The most useful rate data are at constant temperature, under conditions where external mass transfer resistance has been avoided, and with small particles... 

Although methanol from synthesis gas has been a large-scale industrial chemical for 70 years, the scientific basis of the manufacture apparently can stand some improvement, which was undertaken by Beenackers, Graaf, and Stamhiiis (in Gheremisinoff, ed., Handbook of Heat and Mass Transfer, vol. 3, Gulf, 1989, pp. 671—699). The process occurs at 50 to 100 atm with catalyst of oxides of Gii-Zn-Al and a feed stream of H2, GO, and GO2. Three reactions were taken for the process ... 

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

Heat and mass transfer limitations are rarely important in the laboratory but may emerge upon scaleup. Batch reactors with internal variations in temperature or composition are difficult to analyze and remain a challenge to the chemical reaction engineer. Tests for such problems are considered in Section 1.5. For now, assume an ideal batch reactor with the following characteristics ... 

In many operations, for instance in a distillation column, it is necessary to understand the fluid dynamics of the unit, as well as the heat and mass transfer relationships. These factors are frequently interdependent in a complex manner, and it is essential to consider the individual contributions of each of the mechanisms. Again, in a chemical reaction the final rate of the process may be governed either by a heat transfer process or by the chemical kinetics, and it is essential to decide which is the controlling factor this problem is discussed in Volume 3, which deals with both chemical and biochemical reactions and their control. 

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